Solder Joint Quality Control in PCB Manufacturing

Solder joint quality control is a comprehensive, multi-step process in PCB assembly. It involves numerous inspections, tests, and continuous improvements, including reworks. This article traces soldering QC throughout a PCBA’s production stages.

What Defines a High-Quality Solder Joint?

A good solder joint is well-formed, with its appearance and structure within acceptable limits. It looks smooth and silvery without surface imperfections. It also has a concave shape to indicate proper flow and solidification.

Various industry standards explain these requirements, outlining what solder joints should look like. The standards also provide inspection and testing guidelines for internal and external defects.

External faults include opens, solder bridges, tombstoning, and solder balling. On the other hand, internal errors vary from cracks and voids to non-wetted sections. They need specialized equipment to identify.

The Role of Quality Control in Soldering

Why is soldering quality control important? It serves various purposes in PCB electronics. Key functions include preventing early failures and ensuring the board operates reliably. It also enables manufacturers to comply with relevant electronics standards.

Failure Prevention

Faulty soldered connections are more likely to fail in the field. That would lead to electronic device or system malfunctions. Sometimes, solder joint failure leads to total shutdowns.

These occurrences can have devastating effects. For instance, they can cause losses to the user in repairs and downtimes. They also taint the manufacturer’s name and reputation.

Effective solder joint quality ensures the joints are sturdy enough for the application conditions. The joints will not fail due to shock and vibrations or other reasons, such as temperature fluctuations.

Improved Reliability

High-quality joints also provide reliable electrical connections, enabling a printed circuit board to perform its function as expected, depending on the specific application or usage conditions. The board also lasts longer.

Standards Compliance

Product quality standards specify soldering requirements and acceptability criteria for solder joints. The standards outline material and process guidelines and other acceptability rules.

These are usually customer and application-specific, with some industries, such as military and aerospace, requiring sturdier joints. Lead solder fits this requirement alongside rigorous QC steps.

Solder Joint Quality Control Measures

Various departments, including design, assembly, and QC, work together to ensure high-quality joints and final products. The following steps explain how manufacturers conduct QC at different stages of the PCB manufacturing process.

Pre-Soldering Measures

• Board Design: ensuring the design favors soldering by properly spacing components, placing and orienting them for pick-and-place machinery, etc.
  
• Material Selection and Storage: verifying the flux and solder quality and suitability. It must allow proper flow and wetting. Correct storage also prevents contamination.
  
• PCB Preparation: this involves checking the bare circuit board for flaws that may hinder its assembly, such as contaminants, warping, etc.
  
• Component Checking: ensuring assembly components are free from defects, such as broken leads or missing solder balls for SMD packages.
  
• Stencil Verification: the solder paste stencil must be correctly designed for the PCB and in good condition.
  
• Machine Calibration: correctly configuring assembly machines and monitoring equipment. It helps prevent errors that may impact joint quality.
  
• Soldering Environment: controlling assembly conditions by regulating humidity levels, temperatures, and other conditions, such as ensuring dust-free and ESD-safe conditions.
  

In-Process Measures

• Solder Paste Application: ensuring the correct amount of solder paste (not too much and not too little).
  
• Stencil Cleaning: the stencil must be clear of any material before the next paste application step.
  
• Operator Checks: the operator regularly checks the soldering process to ensure everything runs smoothly.
  
• Real-Time Control and Monitoring: implementing automated monitoring and control to ensure various parameters stay within limits.
  

Post Soldering Measures

• Solder Joint Inspections: quality control inspectors examine newly assembled boards for visible solder joint flaws or internal errors.
  
• Quality Check Tests: the QC team ensures that the solder joints are functional and reliable by performing a series of tests.
  

Solder Quality Inspection and Testing Methods

Quality control for soldering involves various inspections and tests. The procedures range from visual checks to reliability testing, where the PCB undergoes simulated environmental extremes.

Joint Inspection

• Visual Inspection: workers inspect assembled boards using their eyes or by looking through a handheld magnifier or microscope.
  
• Automated Optical Inspection (AOI): this method uses special lighting and cameras to obtain high-resolution images of the joints. A computer system projects the images on a screen. It also analyzes them to detect flaws.
  
• X-Ray Inspection: X-rays penetrate through components and soldered areas to detect internal defects. The images it produces can be 2D or 3D, depending on the PCB’s inspection requirements.
  
• Infrared Thermography: this method utilizes infrared technology and thermal conduction to create a solder joint’s image and identify flaws.
  
• Laser-Based Scanning: a laser scans the joints and, using reflected rays, forms images to show defect locations.
  
• Acoustic Imaging: the scanning equipment emits high-frequency sound waves. It then uses the variation in their reflection or passage through the joints to catch faults.
  

Testing Methods

• Electrical Testing: electrical tests check the soldered connections for solder defects like opens and shorts. Key types include functional and ICT tests.
  
• Reliability Testing: this test subjects the board to application-like conditions, stressing the joints to verify their reliability.
  
• Destructive Testing: destructive testing tears down the PCB to check joint strength or access its internal structure.
  

Key Solder Joint Quality Control Standards

Various standardization bodies, such as the IPC, provide soldering and joint quality expectations. PCB manufacturers implement them to ensure circuit boards meet client, market, and application requirements.

IPC Standards

The IPC is one of the most popular standardization bodies in electronics. The organization provides material, process, and verification requirements. The standards divide PCBs into three classes based on quality strictness.

IEC Standards

The IEC publishes standards for soldering procedures and materials, including testing requirements. The requirements include testing procedures for soldering alloys.

Other Standards

Other electronic assembly standards, such as JEDEC, JIS, and ISO, also provide guidelines that aim to ensure high-quality PCB assemblies. Some, like JIS, are regional, while some offer generalized guidelines.

Best Solder Joint Quality Control Practices

Solder joint quality control requires a deep understanding of the entire PCB assembly process. The following are best practices when assembling printed circuit boards.

1. Use the right solder, flux, and paste. Some formulations are more prone to defects, while some may not be appropriate for specific markets. For example, lead solder produces better joints but may not be usable in some regions or PCB types.

2. Ensure proper reflow temperatures and durations. It allows volatile materials to escape and prevents assembly defects, such as voids. It also prevents damage by heat.

3. Maintain utmost cleanliness. It helps ensure the solder and flux flow correctly and covers the desired surfaces.

4. Manufacturers must properly train their workers on the correct use of assembly equipment and best practices. It reduces the chances of errors for flawless connections.

5. Adequately check and test assembled boards for quality using appropriate methods and equipment. The results are crucial to design and production error detection.

6. Take appropriate measures to correct any defects that you identify after inspections and tests. Document the findings, too, for future reference.

Conclusion

Soldering quality control is a crucial process in electronic manufacturing. It ensures electrical and mechanical connections are sturdy and dependable, preventing early failures and their associated risks and losses. In this guide, we discussed how PCB manufacturers implement the QC process in their PCB production.